1. Field of the Invention
This invention relates to electronic circuits and structures, and in particular, to charge-coupled devices (herein CCD) used for line or area imaging.
2. Prior Art
Charge-coupled semiconductor devices were first invented by W. S. Boyle and G. E. Smith. See "Charge-Coupled Semiconductor Devices," Bell System Technical Journal, Vol. 49, page 587, by Boyle and Smith, and U.S. Pat. No. 3,858,232, issued to Boyle and Smith. Since then the development of charge-coupled devices has been described in numerous publications. For example, see the article by Gilbert F. Amelio entitled "Charge-Coupled devices", Scientific American, February 1974, Vol. 230, No. 2, at page 23. It is now well known that line and area imaging devices may be fabricated from arrays of charge storage elements, and such devices are commercially available, for example, products CCD 101, CCD 110, CCD 121, CCD 201 of Fairchild Camera and Instrument Corporation, (herein "Fairchild") assignee of this application. With suitable known structures, the charge stored in the photosites may be transferred into shift registers, and by application of appropriate signals, transferred out of the shift registers to be sensed, amplified, or otherwise utilized by electronic circuits.
Unfortunately prior art charge-coupled imaging devices have been unable to provide an internally generated reference signal indicative of the intensity or half-tone of the light impinging upon the photosites of the CCD, but rather supply information only about the relative intensity of light striking a given photosite compared with other photosites. Only relative information is generated because the video output signal of charge-coupled imaging devices typically is superimposed on a direct current level of several volts, for example +7 to +10 volts. This DC level is usually established by a reset circuit, floating gate potential, or amplifier biasing points and supply voltages. The exact voltage levels for black and white signals of prior art CCD's could be established only by operating the CCD in darkness and in conditions which cause saturation or near saturation by an appropriate optical condition. Even when this procedure was followed, calibration settings made in such tests were subject to drift due to changes in the operating conditions of the device, for example, ambient temperature, supply voltage, or circuit drift.
Further, prior art charge-coupled devices require a relatively large amount of external circuitry devoted to resetting and/or recycling the charge-coupled device to transfer a new set of signals into the shift registers after all of the preceding sequence of data has been transferred out of the shift registers. For example, in prior art line imaging devices a series of counters is utilized to ascertain the number of cycles of transfer signals to be applied to the elements of the shift registers before activating a transfer gate to allow a new set of signals, represented by packets of electrical charge, to be transferred from the photosites into the shift registers. This peripheral circuitry increases the complexity and cost of any system using CCD's.